You either need to remove the stump or kill it off to stop growth by drilling into it with a medium sized wood bit, making holes about an inch or two apart, around quarter to half an inch deep, fill the holes with neat SBK (don't spill it anywhere, it kills anything it touches and sits in the soil), cover with something - an upturned pot, piece of plastic, a paving slab, whatever, and leave to die. If you prefer to have the stump removed, you can hire a stump grinder (very hard to use though), or call a tree surgeon - they usually have someone who'll come out with a stump grinder, remove everything and take it away at the end.

It's reverted to the common acacia, if you don't want a green one dig it out.

Ours died beginning of 2009 and we had no growth whatsoever, was going to pay someone to get it out for us but ended up leaving it. it was 9 years old. this year we looked at it and pushed it and noticed it was quite loose in the ground and with a bit of elbow grease hubby got the whole tree out roots and all,